The present invention relates to acousto-electric filters. More particularly, it relates to an improvement in solid-state tuned circuitry which involves interaction between a transducer device coupled to a piezoelectric material and acoustic surface waves propagated in that material.
In general an acousto-electric filter includes a pair of transducer devices provided on a piezoelectric material in which the acoustic surface waves propagating in the piezoelectric material interact with the transducers. More particularly, as shown in FIG. 1, the surface waves launched from one transducer device A on the surface of the body of the piezoelectric material are caused, in one manner or other, to interact with a second transducer B spaced along the surface from the first transducer A. In the general case, the first transducer is coupled to a source of signals as an input while the second transducer is coupled to a load. The signal energy is translated by the acoustic waves between the two transducers.
The piezoelectric material C is made of single crystal of LiNbO.sub.2, or ceramics of PZT provided as a substrate as shown in FIG. 2, or a layer of ZnO provided over a substrate, as shown in FIG. 3. Each of the transducers includes a pair of electrodes D.sub.1 and D.sub.2 or E.sub.1 and E.sub.2, each of which is formed by a comb-shaped metal film the teeth of which are interleaved with one another. Each pair of teeth in the comb-shaped transducer exhibits resonance for an acoustic surface wave whose half-wavelength is normally equal to the distance between two adjacent teeth. The impedance represented by the wave interaction device is determined by the frequency of the wave to be propagated and also by the capacitance between the electrodes of the transducer device.
In practice, such devices have been demonstrated to exhibit characteristics useable in a number of different applications. In a television or FM stereo receiver, for example, acoustic filter systems have been included in the intermediate-frequency (IF) channel in order to impose a desired IF characteristic with traps or null points at selected frequencies spaced from the IF carrier frequency determined by the structure of components of the acoustic filter system. As another example, an acoustic filter may serve in a frequency-modulation (FM) receiver as the discriminator to perform the necessary function of converting frequency changes to amplitude changes.
While the demonstrations thus far have been highly encouraging, one difficulty encountered has been that, in some system applications, the impedance presented by the wave interaction devices to associate circuitry or other devices is considerably less than that desired for the purpose of obtaining optimum matching and signal transfer characteristics.
In order to eliminate this difficulty, there have been proposed various types of transducer devices.
According to one type of transducer device, the comb-shaped electrodes have an amount of interleaving, that is the distance which the teeth interleave with each other, arranged to be less than that of the conventional type. More particularly, this type of transducer device, as shown in FIG. 4, has interleaving amount T smaller than the interleaving amount T.sub.o of the conventional type, so that the capacitance between the electrodes will be less than that of the conventional type. Other types of transducer devices have another electrode structure, as shown in FIGS. 5 and 6, to reduce the capacitance between the electrodes, in other words, to increase the impedance. The structure of electrodes shown in FIG. 5 has two parts which are connected series and are disposed in such that each of the two parts is in alignment with the direction F of propagation of the wave while the two parts are parallely aligned to each other. The electrode structure shown in FIG. 6 is presented by three parts G.sub.1, G.sub.2 and G.sub.3, each part having same number of pairs of interleaving teeth as the other parts. The three parts are connected in series and are disposed such that each of the three parts is in alignment with the direction F of propagation of the wave while the three parts aligned with each other. In either of the above described cases, the division of the electrodes results in reduction of capacitance between the input terminals.
However, according to first two types of transducer devices shown in FIGS. 4 and 5, the decrease in the amount of interleaving results in an undesirable expansion of acoustic surface waves emanating from each pair of neighboring teeth. Thus, the amplitude-frequency characteristics obtained from these types of transducer devices will deviate from the required characteristics, as shown in the graph of FIG. 9, in which the curve represented by a solid line shows the desired characteristics while the curve represented by a dotted line shows the characteristics obtained from the first two types of transducers.
Furthermore, according to all three types of transducer devices described above, because the amount of interleaving of all the pairs of teeth is equal to each other an unbalance results in the received surface wave vibratory energy which is emanated from the transducer device A. Accordingly, there have been proposed another conventional type of transducer device which varies the amount of interleaving of the pairs of teeth, in a manner as shown in FIG. 7, so as to improve the selectivity characteristics in the transfer function of these devices.
The electrode structure shown in FIG. 8 takes the advantage of the reduced capacitance between the two input terminals by equally dividing the pairs of teeth into a plurality of parts, such as four, as shown in this instance and also reducing the loss of the vibratory energy to be propagated by varying the amount of interleaving of the pairs of teeth.
However, according to this conventional type of transducer device as shown in FIG. 8, the voltage drop at one part of the transducer device, such as P.sub.1, and particularly between the terminals H.sub.1 and H.sub.2, results in a different voltage from the other parts of the transducer device such as P.sub.2, so that the amplitude of the propagation of the surface wave will be deteriorated as shown in FIG. 10. In FIG. 10 curve represented by a solid line shows the desired characteristics while the curve represented by a dotted line shows the characteristics obtained from the above described type of transducer.
Furthermore, in the case where the piezoelectric material is a material having a high dielectric constant such a PZT, the degree of capacitance between the adjacent electrodes will be undesirably increased, while at the same time, the response of the spurious mode will be undesirably increased, so that the vibratory energy of the surface wave is undesirably expanded to deviate from a required characteristics, as shown in FIG. 11. In FIG. 11 the curve represented by the dotted line shows the desired characteristics while the curve represented by the thin solid line shows the characteristics obtained from the above described type of transducer device. In this case, in order to decrease the capacitance, the structure of the pairs of electrodes is separated into a plurality of parts as understood from the foregoing description. However, this separation will not improve the response of the spurious mode to be suppressed.
According to the tests carried out by the inventors, it is found that an incease in the amount of interleaving improves the response of the spurious mode to be suppressed to a desirable amount as shown in FIG. 12, wherein the abscissa represents the maximum amount of interleaving and the ordinate represents the attenuation caused by the spurious response.
Accordingly, a primary object of the present invention is to provide an improved type of a surface wave interaction device which provides an optimum capacitance between the opposite input terminals while propagating vibratory energy without causing an undesirable expansion in vibratory energy.
Another object of the present invention is to provide a surface wave interaction device of the above described type in which the spurious made response is improved.
Yet another object of the present invention is to provide a wave interaction device of the above described type which is simple in construction and can be readily manufactured.